Your TFS data is at risk. Your current backup is not sufficient to guarantee a restore of your TFS environment.
As TFS consists of more than one database (at least tfs_configuration and tfs_collection) there can be transactions that span those databases. Since transactions are not final until complete on a single DB you can get corruption. If you restore to a point in time one db could not have finished the transaction and roll back, while the other completes.
This will cause the TFS restore to fail.
https://msdn.microsoft.com/en-us/library/ms253070.aspx
You must follow the manual backup procedure detailed the documentation to be guaranteed a restore.
Or you can use the built in backup tool in the TFS Administration Console.
NOTE: The SQL Databases, their configuration, and TFS configuration is owned by TFS and should not be changed beyond what has been documented by the product team.